Method of regulating the charging of a set of electrical storage cells, and a facility implementing the method

ABSTRACT

The method and the facility are intended for regulating the current supplied by a charger (3) to a set (1) made up of a plurality of units (2), each of which is provided with an individual interface (5) making it possible to measure voltage, to divert a portion of the charging current, when the voltage measured across the unit is greater than a threshold value, and to supply measurement signals to a controller (4) of the charger. The charging current is reduced as soon as one of the interfaces indicates that it is diverting current, and so long as the charging current supplied is greater than the sum of the currents that can be diverted by the interfaces. The charging current is then maintained at the same level until all of the interfaces indicate that they are diverting current.

FIELD OF THE INVENTION

The invention relates to a method of regulating the charging of a set ofelectrical storage cells, and to a facility for implementing the method.

BACKGROUND OF THE INVENTION

In known manner, such a set is conventionally made up of a plurality ofunits, each of which is identically made up of one or more identicalelectrochemical cells, the units being connected together in series viatheir respective power supply terminals.

Charging such a set by means of a charger connected to the end terminalsof the set generally does not take place uniformly for all of the units.The electrical and chemical characteristics of the units in the same setvary over time and as a function of the operating conditions of the set.This results in particular in the various units of the same set notreaching their respective fully charged levels simultaneously duringrecharging.

It is therefore known that a current can be maintained at the end ofcharging of a set comprising a plurality of units in order to ensurethat all of the units are fully charged including those which are theslowest to become recharged. Unfortunately such a solution to theproblem is not without its drawbacks, because the units that reach theirfully charged levels first continue unnecessarily to pass the chargingcurrent which heats them up and can lead to gases being given off, or,when the units are made up of sealed cells, to excessive pressurebuilding up.

A known solution for remedying this consists in regulating the chargingcurrent for each unit as a function of the voltage across its terminalsby means of a circuit making it possible to divert a progressivelyincreasing fraction of the current away from the unit, at the end ofunit charging, once a voltage threshold representative of the end ofcharging is reached. However, such a solution is not entirelysatisfactory for a set having a large capacity for supplying highcurrents, e.g. such as the sets intended for use in electrically poweredvehicles. The currents required for recharging such sets can also behigh, which means that the currents to be diverted are also high, andthat the power dissipated for regulation purposes at the end of chargingis high.

SUMMARY OF THE INVENTION

The invention therefore provides a method of regulating the currentsupplied by a charger to the end terminals of a set of electricalstorage cells, which set is made up of a plurality of units, each ofwhich is identically made up of one or more identical electrochemicalcells, the units being connected together in series via their respectivepower supply terminals, each unit being provided with an individualinterface including measurement means for measuring the voltage acrossits power supply terminals, means for diverting a portion of thecharging current supplied to the unit, when the measured voltage isgreater than a threshold value, and means for supplying measurementsignals to a controller of the charger.

According to a characteristic of the invention, provision is made forthe controller to reduce progressively the charging current supplied bythe charger to the set, when a current measurement signal is receivedfrom at least one of the interfaces, which current measurement signalexpresses the existence of a current diverted at the interface, so longas the charging current supplied to the set is greater than the sum ofthe currents that can be diverted by the interfaces, the chargingcurrent then being maintained at the same level until currentmeasurement signals are received that express the fact that thediversion means for all of interfaces of the units of the set areoperating.

The invention also provides a facility associating, at leasttemporarily, a charger with a set of electrical storage cells, thecharger being connected to the end terminals of the set, the set beingmade up of a plurality of identical units, each of which comprises oneor more electrochemical cells, the units being connected together inseries via their respective power supply terminals. Each unit of the setis provided with an individual interface which includes means formeasuring the voltage across its power supply terminals, means fordiverting a portion of the charging current supplied to the cell, whenthe measured voltage is greater than a threshold value, and means forsupplying measurement signals to a controller of the charger.

According to a characteristic of the invention, the means for supplyingmeasurement signals from each interface transmit a current signalrepresentative of the charging current diverted at the interface inquestion.

According to another characteristic of the invention, the facilityincludes clock means making it possible to activate successively andperiodically the means for supplying measurement signals from each ofthe interfaces, and in that these means supply their respective currentmeasurement signals to a common transducer serving as an interfacerelative to the controller of the charger.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, its characteristics and its advantages appear moreclearly from the following description given with reference to theaccompanying drawings, in which:

FIG. 1 is a diagram showing a charging facility of the invention forcharging a set of electrical storage cells; and

FIGS. 2A-2D show timing diagrams relating to the charging method of theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a modular set 1 of electrical storage cells, which set isassumed to be made up of a plurality of identical units 2 connectedtogether in series via their respective power supply terminalsreferenced "+" and "-".

All of the units are identically made up of one or more electrochemicalcells which are also substantially identical.

The set 1 can be connected, at least temporarily, to a charger 3 byconnecting the power supply terminals of the charger to the endterminals referenced A and B of the set. The charger may be ofconventional type, and it is assumed to be provided with a controller 4making it possible in particular to adjust the charging current I thatit supplies to the set 1. For example, the controller 4 may be organizedaround a micro-controller (not shown) which includes input-output ports,in particular for receiving measurement signals making it possible todrive the charger as a function of the variation in the state of chargeof the set, and to transmit control signals to the charger proper. Theseelements are known to a person skilled in the art, and they are nottherefore described in any further detail herein. In the embodimentsuggested, an individual interface 5 is assigned to each unit 2 of theset 1, and it is connected to the "+" and "-" power supply terminals ofthe unit 2 to which it is assigned.

Each interface 5 includes, in particular, means enabling it to measurethe voltage V across the "+" and "-" power supply terminals of the unit2 with which it is associated. For example, the measurement means may beconstituted by a differential amplifier 7 having its inverting terminalconnected to a point common to the two resistors 8 and 9 of a dividerbridge connected across the "+" and "-" terminals of the unit, andhaving its non-inverting terminal connected to a point that is common toa Zener diode 10 and to a resistor 11 connected together in seriesacross the above-mentioned "-" and "+" terminals, so that the output ofthe amplifier supplies a control signal to the base of a transistor 12when the voltage V measured across the terminals of the unit exceeds athreshold value VS set by the diode 10.

The transistor 12, of the PNP type, is connected to the "-" terminal ofthe unit via its collector, and via a resistor 13, and it is connectedto the "+" terminal of the unit via its emitter. It makes it possible todivert a portion of the charging current Ic supplied by the charger 3 tothe unit to which it is assigned. This discharging takes place into aresistor 13 which constitutes a current-absorbing element, when thevoltage obtained across the terminals of the unit exceeds the thresholdvalue VS, which value is chosen so as to prevent the set fromunnecessarily and dangerously passing a charging current I that is toohigh at the end of charging.

Each interface 5 also includes means enabling it to supply currentmeasurement signals. In this example, these means supply a measurementsignal i expressing the existence of a diverted current Id diverted atthe interface via the transistor 12 towards the resistor 13.

In this example, the measurement signal i is a current transmitted to ameasurement transducer 14 of the facility, which transducer is shownconnected to the collector of the transistor 12 of each of theinterfaces 5, via a connect/disconnect switch 15 for each interface. Forexample, the switch 15 of each interface may be periodically switched onvia clock means 16 of the facility, which means may, for example,include a sequencer, in this example assumed to be associated with thecontroller 4 of the charger, the sequencer optionally being incorporatedin the controller.

In a preferred embodiment, the sequencer successively controls thevarious interfaces 5, which are connected together in series via acommon control link 17, by sending a stream of control pulses.

Each interface 5 includes activation means 18 known per se andtriggering sending of at least one measurement signal on receiving afirst control pulse after a determined minimum lapse of time. In thisexample, the activation means 18 are assumed to be organized in a knownmanner so as to take into account only the first control pulse that theyreceive, and then transmit any immediately following pulse(s) to theinterface(s) 5 which is/are situated downstream on the link 17, and towhich the pulse(s) is/are addressed.

The measurement transducer 14 receives the measurement signalssuccessively supplied by the interfaces 5. These signals, which are inthe form of current in this example, may, for example, feed ameasurement resistor 19 associated with a measurement device 20, e.g.making it possible to digitize and optionally to pre-process themeasurements taken prior to transmitting them to the controller 4, via alink 21.

At the beginning of charging or recharging the set 1, and when thecurrent I supplied by the charger 3 to the set 1 is at its maximum, asshown by portion I of the FIG. 2A, the voltage across the terminals ofthe units increases over time, as shown for a unit in portion I of theFIG. 2B, as does the voltage across the terminals of the set.

There comes a time when the voltage V across the terminals of at leastone of the units 2 being charged reaches a threshold value VS beyondwhich a portion Id of the charging current flowing towards the unit isdiverted, via the emitter-collector link of the transistor 12 and theresistor 13 of the associated interface 5 because the transistor 12conducts, as represented in FIG. 2C.

The interface then supplies the transducer 14 with a measurement signali which is constituted by taking current at the point that is common tothe transistor 12 and to the resistor 13, the current being taken viacommon sampling path 27, when the switch 15 is switched on via theactivation means 18 of the interface, which means are periodicallytriggered by the clock means of the facility 16.

The measurement signal, which is a pulse signal in this example, istaken into account by the controller 4, optionally after having beenpre-processed and digitized at the measurement device 20. A controlsignal, such as the one shown at level II in FIG. 2D is sent by thecontroller 4 to the charger 3, resulting in a reduction in the currentsupplied by the charger to the set 1.

The voltage across the terminals, both of the units and of the set,decreases accordingly, as shown at level II of FIG. 2B for the unitalready considered.

In an embodiment, this reduction in current is stopped as soon as themeasurement signals obtained from the interfaces express the fact thatthere is no longer any current being diverted at any of the interfaces 5of the units of the set 1, when the respective voltages across the powersupply terminals of all of the units taken individually are all belowthe threshold value VS.

The units then continue to be charged with a current corresponding tothe current defined at the time at which the reduction is stopped.Continuing to supply the charging current I to the set 1 leads to therespective voltages across the terminals of the units 2 increasingprogressively again, and to the set 1 and the charger operating asdescribed above. This operating stage referenced III in FIGS. 2A-2D canbe repeated one or more times as shown in FIGS. 2A-2D.

Towards the end of a charging or recharging operation, a reduction inthe current supplied by the charger would lead to the current becomingless than a threshold value IS corresponding to the sum of the currentsthat can be diverted by the interfaces.

At least some of the units might then be fully charged, and they thensupply a current measurement signal expressing the fact that theirrespective diversion means are operating. Other units still requireadditional charging which leads to the charger continuing to supply thecurrent to the set, as represented in the stage referenced IV in FIGS2A-2D, despite the presence of current measurement signals whichpreviously resulted in another reduction in the charging current.

The controller 4 then causes the charging current I to be stopped, oroptionally causes it to be reduced to a trickle-charging current that ismuch lower than the charging current, when all of the interfaces supplycurrent measurement signals that are characteristic of their diversionmeans operating, with stopping or reducing the current in this wayoptionally being delayed as shown at V in FIG. 2C.

I claim:
 1. A method of regulating the current supplied by a charger (3)to the end terminals of a set (1) of electrical storage cells, which setis made up of a plurality of units (2), each of which is identicallymade up of one or more identical electrochemical cells, the units beingconnected together in series via their respective power supplyterminals, each unit being provided with an individual interface (5)including measurement means for measuring the voltage (V) across itspower supply terminals, means (12, 13) for diverting a portion (Id) ofthe charging current (I) supplied to the unit, when the measured voltageis greater than a threshold value (VS), and means (15) for supplyingmeasurement signals to a controller (4) of the charger, said methodbeing characterized in that provision is made for the controller toreduce progressively the charging current supplied by the charger to theset, when a current measurement signal (i) is received from at least oneof the interfaces, which current measurement signal expresses theexistence of a current (Id) diverted at the interface, so long as thecharging current (I) supplied to the set is greater than the sum of thecurrents (I) that can be diverted by the interfaces, after which thecharging current (I) then being maintained at the same level untilcurrent measurement signals are received which indicate that thediversion means for all interfaces of the units of the set areoperating.
 2. A battery charger system associating, at leasttemporarily, a charger (3) with a set (1) of electrical storage cells,which set is made up of a plurality of identical electrochemical units(2) connected together in series via their respective power supplyterminals (+, -), the charger being connected to the end terminals (A,B) of the set, each unit of the set being provided with an individualinterface (5) which includes means (7 to 11) for measuring the voltage(V) across its power supply terminals, means (12, 13) for diverting aportion of the charging current supplied to the cell, when the measuredvoltage (V) is greater than a threshold value (VS), and means (15) forsupplying measurement signals to a controller (4) of the charger, saidsystem being characterized in that the means for supplying measurementsignals from each interface transmit a current signal (i) representativeof the charging current (Id) diverted at the corresponding interface. 3.A battery charger system according to claim 2, characterized in that itincludes clock means (16) making it possible to activate successivelyand periodically the means (15) for supplying measurement signals fromeach of the interfaces, and in that these means supply their respectivecurrent measurement signals to a common transducer (14) serving as aninterface relative to the controller (4) of the charger.
 4. A method ofcharging a series of electrochemical storage cells in a batteryconnected to an adjustable charging current supply, each cell having avoltage sensor and a current diverter circuit monitored by a controller,said method comprising the steps of:applying a charging current from theadjustable charging current supply to the battery; diverting a portionof said current around each cell in proportion to the amount by whichsaid corresponding cell voltage sensor detects a cell voltage thatexceeds a predetermined threshold; generating, for each cell, a currentsignal proportional to the amount of current being diverted (Id);monitoring said current signals for each cell; and signalling from saidcontroller to said charging supply to reduce the charging currentwhenever at least one of said monitored current signals indicates adiverted current (Id).
 5. The method of claim 4 wherein the sequence ofmonitoring the current signals and signalling the charging currentsupply is repeated as long as said charging current is greater than apredetermined current, and then said charging current is maintained atthe same level until current signals from every said cell indicate acurrent being diverted.
 6. The method of claim 4 wherein the step ofmonitoring said current signals further comprises the stepof:sequentially activating a switch associated with each cell, wheresaid switch temporarily connects the diverter circuit to a transducerfor measuring the current being diverted around said cell.
 7. The methodof claim 6 wherein said step of monitoring said current signals furthercomprises the steps of:sending a pulse train from said controllerthrough an activation circuit, where said activation circuit includes,for each cell, a serially connected activation means for activating acurrent-sampling switch in the corresponding cell, each activation meansbeing activated only once during each said pulse train, and measuring asignal pulse corresponding to each cell as its current-sampling switchis activated.